Here's a post about it:
Apparently only happens with seatbelt unbuckled. From what I can determine, the brakes are applied if over a certain speed, but if stopped and trying to move in reverse, it goes to park. I don't think the transmission would be damaged.
Owner's Manual (p.68):
P is engaged automatically:
* If the safety belt is unbuckled, the driver's door is opened, and the brake pedal is not pressed while the vehicle is stationary and transmission position R or D is engaged.
2013 x35i (Space Gray, MSport, CW/CWII, DHP, Premium, Premium Sound, Technology, BMW-Apps)
Retired:'09 C300 4Matic, '09 GTI, '06 750i, '05 C55, '04 R32, '03 M5, '02 GTI, 01 M5, '00 M-Coupe, '00 540i/6, '99 M3 Coupe, '98 M3 Coupe, '96 328i, ..., '86 4000CS Quattro, ... too many to list
Last edited by noka; 03-08-2013 at 10:44 AM.